1. Technical Field
The present invention pertains to a low-pass optical filter used in an electronic imaging system to reduce aliasing or undersampling artifacts.
2. Background Art
An electronic imaging system typically produces a signal output corresponding to a viewed object by spatially sampling an image of the object in a regular pattern with an array of photosensitive elements, such as, for example, a charge-coupled device (CCD) solid-state image sensor. In such an imaging system, it is well-known that components in the object which contain fine details create frequencies too high to be analyzed within the sampling interval of the sensor and contribute to the amplitudes of lower frequency components, thereby produce imaging errors commonly referred to as aliasing or undersampling artifacts. In particular, if the spatial detail being imaged contains a high frequency component of a periodicity smaller than the pitch of the photosensitive picture elements of the solid state image sensor, the subsequent detection of this high frequency component tends to result in a spurious signal due to aliasing.
In general, the electronic imaging system can minimize aliasing if its optical section has a frequency response that cuts off, or filters out, the higher frequency content of the object being imaged. As a result, the optical section generally employs an optical low pass filter to substantially reduce the high frequency component contained in the spatial detail of the image received by the image sensor. Thus, prior art design of electronic imaging systems involves a trade-off between image sharpness and the susceptibility of the imaging system to aliasing distortions or undersampling artifacts.
To limit these artifacts, an optical filter, for example, a birefringent blur filter, has become a common component in consumer color video cameras. U.S. Pat. Nos. 4,101,929 and 4,896,217 show typical examples of such filters. Such a filter is usually placed between a lens and the image sensor to provide a low-pass filter function which reduces the spatial frequency content of the object at frequencies above the Nyquist frequency of the photosensitive elements. This makes the imaging system less susceptible to aliasing distortion. For example, for sensors which have equal pixel densities in each of the sensed colors, thus each of the sensed colors have the same Nyquist frequency, an achromatic low-pass, or “blur,” function is effective in minimizing aliasing distortion. Such a function can readily be provided by a birefringent filter.
The birefringent blur filter is typically composed of filter plates manufactured from a crystalline material like quartz that exhibits a dual refraction effect when the crystal axes of the filter plates are oriented at an angle with respect to the plate surface. In this orientation, a randomly polarized ray of light passing through such a filter plate emerges as two separated polarized rays. The combination of several of such plates produces a multiple spot pattern from each incident point in the image. If this spot pattern distributes light energy over multiple photosensitive elements, then the blur effect is obtained. This will limit the optical transfer function of the system at spatial frequencies above the Nyquist frequency of the photosensitive elements.
One of the most common blur filters in the prior art is the four spot filter made of crystal quartz plates. Quartz is a uniaxial crystal, namely, it has one unique direction called the crystal optical axis so that when light propagates along this so called ordinary direction, the index of refraction for all polarization directions is the same and it is designated “No.” For light propagating perpendicularly to this axis, the so called extraordinary direction, the index of refraction is “Ne.” If a crystal 10 is cut as shown in FIG. 1, so that the crystal optical axis 12 is at an angle α to a line normal to the input facet 14, then an incoming unpolarized ray 16 will be split into two rays. This is known as double refraction. One ray will continue unaffected and is called the ordinary ray 17. The other ray, called the extraordinary ray 18, will emerge from the crystal displaced from the ordinary ray. The separation between the rays is given by:
 S=t*(No2−Ne2)*Tan(α)/(No2+Ne2*Tan2(α))  Equation 1:
                No is the ordinary index of refraction        Ne is the extraordinary index        For crystal quartz at 546 nm:                    No=1.5462 and Ne=1.5554                        t is the plate thickness        α is the angle between the crystal optical axis and the normal to the surface of the plate Maximum separation (S) occurs when the angle α is about 45°.        
One way of making a four spot filter 20 is by using two crystal quartz plates 21 and 23, double refractors, as shown in FIG. 2, with a quarter wave retarder 22 between them. A similar four spot filter is suggested by Sato in U.S. Pat. No. 4,626,897. FIG. 2 shows the three pieces separated, but ordinarily, they are cemented together. The first double refractor 21 separates the beam into two beams with, for example, a separation in the vertical direction. The retarder 22 converts the two linearly polarized beams 17 and 18 into circularly polarized beams 27 and 28 which are then split in the horizontal direction by the second double refractor 23.
Using Equation 1, if the pitch of a given CCD detector size is 9 μm, the preferred separation S for a square four spot filter is equal to 9 μm. Substituting S=0.009 mm in the equation above, the required plate thickness t for the two double refractors is 3.04 mm. (This does not include the thickness of the retarder in between). This large thickness of 3.04 mm is due to the small birefringence, namely the small difference between the indices of the crystal quartz, No−Ne=0.0092.
FIG. 3a shows another way of producing a square, four spot blur filter 30 according to the prior art. In this case, a first double refractor 21 is used as in FIG. 2 to separate the spots, for example in the vertical direction, as shown in FIG. 3b. This shows the spots at a detector plane 24 when only the first double refractor 21 is used. Referring again to FIG. 3a, the second double refractor 33 has a crystal axis in a plane 33a tilted at 45°. The thickness of the second double refractor 33, t2 is smaller than the thickness of the first double refractor, t2=0.707t1. FIG. 3c shows spots that would be produced by the two beams at the detector plane 24 and their polarization in the coordinate system of the second double refractor. The second double refractor 33 splits each of the spots as shown in FIG. 3d at the detector plane.
Referring again to FIG. 3a, the third double refractor 36 has a plane 36a at 90° to the plane 33a of the second double refractor 33, and has the same thickness as the second double refractor. The third double refractor splits the beam again and a square pattern is achieved as is shown in FIG. 3e. The double refractors are cemented together to reduce reflection losses. The filter assembly is aligned so the square pattern is parallel to the coordinates of the CCD which comprise an image sensor located at detector plane 24.
The filters discussed above, however, suffer from the drawback that the thickness required to achieve the desired blur requires a lens with a long, back focal distance in order to make room for the blur filter in the optical path. Space limitations often do not allow such an optical structure, and lens design becomes unduly complicated. In most digital cameras, space is at premium and there is no room for a thick filter. For example, in cameras using a flipping mirror, the space in front of the detectors is needed for the mirror assembly. Also, when a digital camera which was originally designed as film camera is modified for use with a CCD detector, in addition to the mechanical problems associated with accommodating a thick blur filter, a lens designed for film may not perform as well with a thick filter, which may introduce aberrations. In these cases, a thinner filter is useful, which may fit the space constraints and will introduce less aberrations than a thick filter.
Another problem with current art four spot filters is that they are commonly made of three pieces, either two double refractors and a retarder as in the Sato, U.S. Pat. No. 4,626,897; or as in FIG. 3a, which uses three double refractor plates without a retarder. A filter made of two elements would be less complex and less expensive. Watanabe U.S. Pat. No. 3,784,734 proposed a blur filter made of two double refractors for color image pickup using a striped filter array. Watanabe only needed to split the image spot into three or four spots along one direction, perpendicular to the stripes direction. In most modern CCD images, a color filter array used is commonly a two-dimensional array of color filters as in Bayer U.S. Pat. No. 3,971,065, and a blur filter is required to split the imaging spot into four or more spots arranged in a two-dimensional pattern, so the Watanabe design would not be satisfactory.
It is also well known in the art to use a phase diffraction grating as a frequency selective filter to produce an image blur. For example, as shown in U.S. Pat. No. 4,998,800, the periodicity of an image of a diffraction grating projected onto a solid state image sensor is selected to be a multiple of the periodicity of the photosensitive picture elements, and a blurred image is obtained. This type of filter, however, suffers from the drawback that, instead of producing a tightly controlled pattern over a few photosensitive elements, it spreads light over many interference fringes (orders) theoretically out to infinity. In addition, it is difficult to control the energy distribution in the fringes in order to obtain an acceptable blur function covering a designated number of pixels. Moreover, the energy distribution is dependent upon wavelength.
As can be appreciated from the foregoing remarks, there is a need in the art for a physically thin blur filter that is inexpensive and relatively simple to manufacture, yet which produces a tightly controlled blur pattern that is not dependent upon polarization techniques. As an alternative to the birefringent blur filter and the phase diffraction grating, U.S. Pat. No. 4,989,959 discloses a pyramidal structure comprised of four wedges which divide incident light into four quadrants so that light from the same image point impinges, on the average, on several photosensitive elements in the image sensing device. To produce four abutting facets at identical angles on a single piece of material, one facet would ordinarily be machined or ground into a single piece of material, the piece would then be cut into sections, and the sections glued together to form a piece shaped like a pyramid. This filter produces the desired spots at the CCD plane when the lens is at focus, however, as the lens is slightly defocused, the spots tend to blend and the anti-aliasing efficiency is diminished. Further, this anti-aliasing filter is positioned at the lens pupil (or at the exit or entrance pupil); not in proximity to the imager. By placing the filter at the pupil, an auto focusing system, if present, may be confused. Also, the lens has to be designed to accommodate such placement of the filter. Since the filter is focal length dependent, it will not work with a zoom lens.
Commonly assigned U.S. Pat. Nos. 5,322,998 and 5,438,366 disclose a conical blur filter that reduces undersampling artifacts by generating a blurred image produced by limiting higher spatial frequencies of incident image light. The blurred image takes the form of a circular blur pattern, for each input point source. Depending upon the shape of the filter, the circular blur pattern may be an annular blur pattern covering a two-dimensional array of photosites, or the central part of the pattern may be filled in with blurred light. Although this is a very good filter if there is ready access to the pupil, it too is located at the aperture and may confuse an auto focus system, if there is one, and will not work with a zoom system.